Generally, a rectifier is needed in a switching mode power supply system to provide a DC output voltage from an input voltage. An SR can improve the power conversion efficiency by reducing the conduction losses in the rectifier device. Recently, metal oxide semiconductor field effect transistor (MOSFET) devices have been used as the SR due to low conduction losses of a properly selected MOSFET in these applications.
In MOSFET SR circuits, turn-on and turn off of the SR transistor must be synchronized with the control signal of the primary switch. Taking the flyback converter as an example in this text, the SR transistor is turned “off” by a gating signal (a potential at the gate of the MOS transistor against the source terminal) during the time that the primary switch is turned “on” or the secondary current reaches zero, and turned on when the primary switch is turned off. In addition, on the primary side of a switching converter, a primary gate, usually a large power MOSFET transistor is used to control current flowing through a primary coil. Typically, the primary switch or transistor is turned on and off by a pulse width modulated (PWM) signal with a fixed duty cycle or a pulse frequency modulated (PFM) signal.
A “dead time” should be established between switching cycles so that so-called “shoot through” will not happen. Shoot through means that the converter output is shorted through the SR transistor and the transformer secondary side winding and the output capacitor is unnecessarily discharged. Additionally, this shoot through may cause over current at the primary side and disturb proper operation of the converter. Besides these, the system efficiency and EMI performance may be negatively influenced, as well. To prevent this shoot through effect, timing circuitry is utilized to ensure that the gating signal to the SR MOSFET transistor is switched off before transformer secondary side voltage polarity switches.
Conventional approaches to the secondary side switching problem are to sense the switching of the primary gate (typically a transistor is used as the switching element) on the secondary side of the converter. Control circuitry then uses that signal to control the gate of the SR at the secondary side to create the dead time, so that the two transistors are not turned on at the same time. Conventionally, an impulse transformer is used to provide the primary gate switching signal to the secondary side of the circuit. Alternatively, the turn on for the primary can be determined by sensing the voltage across the drain to source of the SR transistor at the secondary side of the converter. By observing a rising edge of the drain-source voltage of the SR, the primary gate turn on can be detected.
A first disadvantage of these approaches is that they require added components, and reliability of operation of the secondary side power regulation circuits is not assured over a range of conditions which may occur in different voltage converter applications. In one known approach, a control circuit is provided on the secondary side of the converter that uses a rising edge detector across the SR to determine the primary gate turn on. However, this circuit is not able to distinguish an oscillation or noise event from a true primary switch turn on event. In a known enhancement to this approach, a slew rate monitoring detection circuit is used on the secondary side to detect a true primary gate turn on and distinguish oscillations or noise events. However, this slew rate monitoring circuit adds components and assumes a certain slew rate, which limits the applicability of this approach.
A second disadvantage of conventional approaches is that under certain circumstances, a secondary side control circuit that turns on the SR MOSFET based on the voltage sensed across the source-drain terminals of the SR MOSFET will erroneously turn the gating signal on a second time within a cycle period, due to oscillation or noise in the drain to source voltage signal being sensed. These voltage oscillations are typically present due to parasitic elements that are unavoidable in the circuitry. The erroneous second turn on of the SR MOSFET results in improper circuit operations.
A continuing need thus exists for efficient and reliable circuit and methods to control the operations of an SR in a switching converter circuit.